Compare two wall, roof, or floor insulation quotes using whole-assembly U-values or recent bills, complete project and defect costs, payback, sensitivity, and 10- or 20-year NPV.
Input rule: Zero-cost defaults are not market averages. Enter recent bills and tested quote data, or load the synthetic example only to inspect the model.
The thermal method estimates conductive envelope savings and caps them at recent use. The bill method applies realization to a contractor savings claim.
Enter measured opaque-envelope area and an existing U-value supported by survey or performance evidence. Leave excluded surfaces at zero area.
| Surface | Project area | Existing U-value |
|---|---|---|
| Exterior wall | m² | W/m²·K |
| Roof / top-floor ceiling | m² | W/m²·K |
| Floor / lowest floor | m² | W/m²·K |
Use a consistent recent 12-month period. Convert heating fuel to a kWh-equivalent basis and derive effective prices from the same bill scope.
Verify location, base temperature and period
Use zero if cooling is excluded
Convert fuel to a consistent energy basis
Conservative allowance for bridges, airtightness and behaviour
Compare two quotes with matching inclusions and exclusions. Enter only approved or otherwise confirmed support, never an intended application amount.
The model uses a fixed 20-year horizon. Keep escalation and discount assumptions consistently nominal or real; sensitivity changes savings only.
Check your inputs
For the thermal method, enter an area for at least one wall, roof or floor surface.
A promise that wall or roof insulation will reduce heating bills is not enough for an investment decision. Savings depend on measured opaque-envelope area, existing and improved whole-assembly U-values, local heating and cooling degree days, system efficiency, thermal bridges, airtightness, workmanship and occupant behaviour.
This calculator compares two wall, roof or top-floor ceiling, and floor insulation options on the same baseline. Use the thermal method to estimate conductive load reduction, or use recent bills to stress-test a contractor savings claim. Then include actual construction, ancillary work, maintenance, a one-off defect repair, confirmed service life, escalation and discounting to see simple payback, discounted payback, and ten-year and twenty-year net present value.
Zero-cost defaults are not Korean market averages or recommended prices. Enter recent bills, measured areas, tested performance and itemised quotes. The synthetic example demonstrates the model only.
Windows need whole-window U-values, frame and glazing performance, solar gain and airtightness, so they belong in the separate window-replacement calculator. This page covers opaque exterior walls, one effective roof or top-floor ceiling boundary, and floors exposed to outside air, ground or unconditioned space. Do not count a window inside wall area or count both a roof and an already insulated ceiling when they represent the same thermal boundary.
A U-value describes heat flow through one square metre of an assembly for each kelvin of temperature difference. Lower is better. Multiplying the reduction from existing to improved U-value by area gives the reduction in envelope heat-loss coefficient. Add the active wall, roof and floor contributions to obtain ΔUA, then combine it with heating or cooling degree days and twenty-four hours per day.
ΔUA = Σ[area × (existing U − improved U)]
heating load saved = ΔUA × HDD × 24 ÷ 1,000
cooling load saved = ΔUA × CDD × 24 ÷ 1,000
heating input saved = load saved ÷ efficiency × realization
cooling electricity saved = load saved ÷ COP × realization
Purchased-energy savings are capped at recent annual use so an aggressive U-value or degree-day assumption cannot save more energy than the household bought. A zero usage entry therefore produces zero savings for that energy source. Gas, kerosene and LPG must be converted to a consistent kWh-equivalent basis using supplier calorific data or an energy assessment.
When existing construction is unknown, multiply annual heating and cooling use by matched effective prices to establish recent annual energy cost. The calculator applies each option savings claim and the common realization factor to that cost. This does not certify the claim. It asks whether a conservatively reduced version of the claim could recover the complete owner cost.
A large gap between the thermal and bill methods is a useful diagnostic. Recheck net area, whether the quoted U-value covers the complete assembly, degree-day base temperature, fuel conversion, bill period, fixed charges and any savings caused by equipment or behavioural changes rather than insulation.
Insulation thickness alone does not establish the improved whole-assembly U-value. Existing layers, fixings, gaps, junctions, moisture and workmanship all affect delivered performance. Check exactly what the test report, calculation and specification cover.
| Output | Meaning | Horizon |
|---|---|---|
| Net initial cost | Construction + ancillary work − confirmed support | Year 0 |
| Year-one net cash flow | Energy savings − maintenance − any year-one repair | Year 1 |
| Simple payback | Undiscounted recovery with flat first-year savings and entered repair costs | Up to 20 years |
| Discounted payback | Recovery after escalation and present-value discounting | Up to 20 years |
| Ten-year and twenty-year NPV | Present value of net annual flows − net initial cost | 10 and 20 years |
Simple payback is easy to communicate but omits discounting, escalation and benefits after recovery. Discounted payback adds timing but still ignores later value when ranking alternatives. For two mutually exclusive options, review twenty-year NPV first, then check initial cash, service life, defect exposure and the low-savings scenario.
A star identifies the higher twenty-year NPV under the entered assumptions. If both values are negative, the starred option is merely less negative. It does not mean that either project is economically profitable or that non-energy benefits have been valued.
Assume 100 square metres of wall, an existing U-value of 1.0 W/m²·K, an improved U-value of 0.5 W/m²·K, 2,000 heating degree days, no cooling, and one-hundred-percent heating efficiency and realization. ΔUA is 50 W/K and annual heating energy saved is 2,400 kWh. At KRW 100 per kWh, first-year savings are KRW 240,000.
With KRW 1,000,000 net initial cost, no maintenance or repairs, zero escalation, zero discount and twenty-year service life, both paybacks equal about 4.17 years. Ten-year NPV is KRW 1,400,000 and twenty-year NPV is KRW 3,800,000. Adding a KRW 200,000 repair in year three moves payback to five years and reduces twenty-year NPV to KRW 3,600,000.
First-year saving
KRW 240,000
Simple payback
About 4.17 years
20-year NPV
KRW 3,800,000
This deliberately simple case verifies the arithmetic. It is not an average Korean home, price, U-value, savings rate or recommended specification. Real buildings add orientation, solar gain, infiltration, ventilation, internal gains, moisture, changing set points and workmanship.
Scaffold, lifts, hoisting, weather protection, road use and safe occupied routes
Existing finish or insulation removal, sorting, haulage, disposal and extra-load rates
Cracks, corrosion, decay, leak damage, paint, wallpaper, trim and service relocation
Window reveals, slab edges, columns, beams, eaves, corners and penetrations
Vapour-control position, jointing, drying, humidity management and ventilation upgrades
Survey, U-value calculation, condensation review, details, inspection and verification
Furniture moves, temporary accommodation, cleaning and genuinely incremental lost use
Cracking, detachment, leaks, condensation, staining and owner cost outside warranty
An unresolved item should remain a quote question rather than silently becoming zero cost. The one-off defect repair is a stress input, not a prediction that failure will occur. Set it separately for each method using warranty exclusions, site exposure and professional advice.
Exterior insulation may improve thermal-bridge continuity but can add scaffold, facade, boundary, eave and window-junction work. Interior insulation may preserve the facade but reduce room area and increase service relocation and interstitial-condensation concerns. Compare complete ancillary scope and defect reserves, then see whether the ranking survives the low-savings case.
Separate the roof replacement that is already required from the incremental cost caused by the insulation decision. Otherwise the insulation payback can look artificially poor or artificially attractive. Use the roof lifecycle calculator for the complete covering decision and this page for consistently defined insulation alternatives.
An intended application is not confirmed support. First test owner-funded economics at zero support. Enter an amount only after the responsible agency confirms eligibility and approval. An interest-supported loan is not a cash grant, so do not deduct the full loan principal as support.
Sources were checked on August 24, 2026. The current Korean Building Energy Conservation Design Standard is MOLIT Notice No. 2026-360, effective July 8, 2026. Article 2 connects exterior walls, top-floor ceilings or roofs, and lowest floors to prescribed U-value or insulation-thickness methods. Articles 5 and 6 address envelope definitions, area-weighted U-values, evidence, vapour control, continuity and airtight joints.
Those regional and component values support regulatory design review for work within the standard. They are not measured existing U-values or automatic investment targets for every older home. The calculator therefore selects no legal preset and makes no compliance decision. Confirm the actual building, work scope, permit or filing position with the competent authority and qualified professionals.
The 2026 Energy Use Rationalization financing guidance includes insulation-related work but has business, ESCO, public and nonprofit eligibility structures. It is not used as a generic homeowner grant. The GreenTogether portal reported the restart of the 2026 private-building green-remodelling interest-support program and free consulting, but current eligibility, limits, interest terms and approval still require the live notice and agency confirmation.
Obtain degree days from the Korea Meteorological Administration Data Portal with a recorded station, base temperature and period. Economic structure follows the screening and lifecycle principles in NIST Handbook 135e2025: payback is useful for screening, while present-value measures should support comparison between alternatives. Neither source supplies a Korean market price or guaranteed service life for this calculator.
Prefer an assembly calculation, heat-flow measurement or energy audit. If an age-based estimate is unavoidable, document it as an assumption and rerun a wider sensitivity range.
No. Hot water, cooking, distribution loss and fixed charges may be unrelated. Match the scope of use and effective price, then use conservative realization and the recent-use cap.
No. It demonstrates the model only. Replace it with an assessor value or a documented performance basis that reflects bridges, airtightness, behaviour and workmanship.
Do not treat an application as confirmed. Test zero-support economics first and enter only an approved amount that actually reduces owner cost. Never deduct an interest-supported loan principal as though it were a cash grant.
The entered savings and costs do not recover initial investment before the confirmed life ends. Record comfort or non-energy benefits separately rather than forcing them into energy savings.
No. Vapour movement, vapour-control position, indoor humidity, ventilation, bridges and existing moisture need hygrothermal review. Interior insulation deserves particular attention.
No. NPV covers only entered energy and cash flows. Structure, fire, moisture, buildability, contract risk, warranty enforceability and available cash remain separate decisions.
Even a strong economic result does not replace thermal-bridge, airtightness, vapour, existing leak, moisture, fire and structural review. After construction, retain indoor-temperature and matched energy-use records. A material performance gap should trigger review of controls, ventilation, airtightness and workmanship rather than an assumption that weather alone explains it.
Consistent area, U-value and scope assumptions reveal the difference between a low headline quote and a recoverable project. Review the low-savings case, then take the result to an energy assessor and both contractors to close missing performance and cost evidence in writing.